The frequency of alleles can range from zero to one (no presence to presence of nothing but, respectively).

Alleles with a frequency of zero either have gone extinct or never existed. Alleles with a frequency of one are said to be fixed. Frequencies found between zero and one are associated with polymorphisms, that is, where more than one allele exists at a single locus within a population.

For the sake of review, recall that a locus is the location of a gene on a chromosome while an allele represents a given genetic variant of a gene. That is, base sequence exists at specific locations within larger nucleic acidsequence and this sequence can vary.

The sequence is a gene, its location is a locus, and variation in that sequence is allelic.